All planes mentioned above are connected via interfaces (indicated
with "Y" in Figure 1. An interface may take multiple roles depending
on whether the connected planes reside on the same (physical or
virtual) device. If the respective planes are designed so that they
do not have to reside in the same device, then the interface can only
take the form of a protocol. If the planes are collocated on the

It should say:

All planes mentioned above are connected via interfaces (indicated
with "Y" in Figure 1). An interface may take multiple roles depending
on whether the connected planes reside on the same (physical or
virtual) device. If the respective planes are designed so that they
do not have to reside in the same device, then the interface can only
take the form of a protocol. If the planes are collocated on the